Transfer RNAs (tRNAs) associate with genomes of RNA tumor viruses and thereby serve to prime DNA synthesis. We plan to study the primer binding sites in the genomic RNAs of avian sarcoma viruses (which bind tRNATrp) and murine leukemia viruses (which bind tRNAPro). In addition we plan to study the structures of the tRNA binding sites of several other large RNAs such as a 30S mRNA of mouse cells and 28S rRNA. The primer binding sites of virus-specific mRNAs isolated from infected cells will also be studied with regard to sequence and location.